Ophthalmic glass



, 2,699,398 .P'atented. Jan. 11 1 955 OPHTHALMIC GLASS Wiliiam-H.-Armistead,-Corning, N. Yi, assignor to Corniug GlassWorks, Corning, N. Y., a corporation of I New-York No Drawing. Application February 18, 1952,

. .SerialNo. 272,243"

5 Claims. (Cl; 106'-53)' This invention relates to glasses having refractive indices (u i-in the neighborhood of 1.63 to= l.66 and dispersive indices (V) of about 40 together with sufiiciently low softening points and sufiicientlyhigh. chemical durability and resistance to devitrification for utilization as the high-index segments in multifocal ophthalmic lenses. In the production of multifocal lenses such as bifocal lenses, it. is. customary to. utilize a bu tton.- orsegment having a softening. point.40.to.. 50 vC. lower than that of the crown sothat the segment can soften and fuse to* the crownrwithout destroying. thecurvature, that has been ground into the crown. For the-ordinary standard white optical or hard crown glasses, which generally have a softening-point of about 725 to-t730" C., various button glasses having the desired lower softening point are'readily'available.

Many of the recently introduced tinted crown glasses have considerably lower softening points, however, the softening points of certain greenish crown glasses being as low as 670 C.; and previously existing glasses have not-been entirely satisfactory for use in bifocal segments for many of these tinted glasses in that, generally, either the softening point spread is not sufficient or such previous glass does not possess the requisite physical characteristics, especially where a refractive index of about 1.63 to 1.66 and a dispersive index of about 40 (approximately 38 to 42) are required. As a matter of fact, so so far as applicant is aware, there has not been heretofore available a glass having refractive and dispersive indiccs within the indicated ranges and also having a sufiiciently low softening point for use as a button glass with the above-mentioned green-tinted crown glasses.

The indicated physical characteristics together with a reasonably low softening point can be obtained, for example, in certain prior barium and barium flint glasses. Such glasses, however, exhibit a marked tendency to devitrify during the remolding operation necessary in the production of bifocal lenses. To eliminate such devitrification, it is customary to include in the glass composition a substantial amount of CaO, which has the undesirable effect, however of raising the softening point.

Lowering of the softening point of such a glass can be accomplished by increasing the proportion of fluxes such as PbO, alkali metal oxides, and B203. Such a change in the glass composition, however, adversely affects its acid durability, which should be sufiicient so that the glass is not spotted by weak acids such as those found in grapefruit.

Bifocal segments and lenses containing the same-are conventionally washed with alkaline solutions for removal of the pitch utilized in attaching them to blocks for polishing, and the alkali durability of button glasses should thus be sufiicient to enable them to Withstand any alkaline attack during such treatment. To insure this resistance, ZrOz is usually added to the glass composition in substantial amount. Such addition, however, also tends to raise the softening point objectionably.

I have now discovered that these oxides, with the exclusion of CaO, can nevertheless be so compounded and balanced as to yield glasses which have the desired physical and chemical properties and which are particularly suitable for the production of bifocal segments for use with the softer crown glasses. These new glasses are free from calcium and comprise essentialy 31 to one: alkali metal-oxide 1n theeindtcated' proportiorrzselected from the group consistingxofiupto 8%: K; up to" 9%;

NazO and up to 2.5% Li2O,. the. selected: alkali: metal oxide including at least one oxide selected: from the group consisting of NazO and. LizO, the total of the ,alkali metal oxides being 6 to 9%, and.3 to 6% BzOx; the totalzof' the essential constituents"beingatlcastt98%: If desired, up to 2% TiOzmay be. included for the. purpose, of. raising the refractiveindex of the' glass;

The following compositions calculated in' percent by weight from their respectiveabatches will further-illustrate the invention:

A fining agent such as AS293, SbzOa orrNazSOt pref-1 erably is: added, to-the batches; for; the aboyesglasses in an amount. ranging from. 0.1. to. 095% of. the. finished glasses on the oxide basis, but such amounts have substantially no effect upon the physical and chemical properties of the glasses. Unavoidable traces of impurities such as Fe203, MgO and CaO may also be present without substantial effect on the properties of the glasses.

Variations in the proportions of the essential constituents of the present glasses should be confined within the limits set forth above for the following reasons:

An excess of SiOz in the glass raises its softening point and lowers its refractive index objectionably while a deficiency of SiOz causes the glass to have too low an acid durability.

An excess of PbO likewise lowers the acid durability of the glass too much and also its dispersive index while a deficiency of PhD raises the softening point and lowers the refractive index. An excess of BaO causes devitrification of the glass and raises its softening point objectionably and a deficiency of BaO makes the dispersive index 0 of the glass too low. If the total PbO and BaO exceeds An excess of ZrOz will fail to dissolve or will precipitate in the glass but at least 5% is essential to maintain suitable alkali durability.

At least 6% total alkali metal oxide is required to prevent the softening point of the glass from being too high and its expansion coefficient from being toolow but such total should not amount to more than' a b out 9% in order to avoid too high an expansion coefiicient or too low an acid durability. It is preferable to use NazO because it is cheaper and has a better fiuxing power than K20. Glasses containing little or no NazO, however, advantageously contain Li2O to offset the lower fluxing power of K20.

An excess of H203 lowers the acid durability of the glass objectionably while a deficiency of B203 results in a tendency towards too high a softening point and too low a dispersive index. The dispersive index is also undesirably lowered by an excess of TiOz.

The presence of alkaline earth metal oxides other than BaO, and particularly those of low molecular weight such as MgO and C210, is to be avoided except as trace impurities on account of their undesirable effect on the softening point and acid durability of the present glass.

41% SiOz, 18 to 33% PbO, 12 to 25% BaO, the total Finally, the total of the essential ingredients should be; at least 98% in order to provide sufficient acid dura- 1 ity.

The above-described ophthalmic glasses may if desired be hated by including therein suitable glass coloring hxides. Glasses of this type which are colored green to match the green tinted crown glass are described in my pending application Serial No. 272,244 filed concurrently herewith.

' As used herein, the softening point of a glass is that temperature at which the glass has a viscosity in the neighborhood of 4.5 X 10" poises.

- What I claim is:

'1. A calcium-free glass comprising essentially by weight 31 to 41% Si02, 18 to 33% PbO, 12 to 25% BaO, the total PbO and BaO being 40 to 50%, 5 to 10% ZrOz, at least one alkali metal oxide in the indicated proportion selected from the group consisting of up to 8% K20, up to 9% NazO and up to 2.5% LizO, the selected alkali metal oxide including an oxide selected from the group consisting of NazO and LizO, the total of the alkali metal oxides being 6 to 9%, and 3 to 6% B8233, the total of the essential constituents being at least 9 a.

2. A calcium-free glass comprising essentially by weight 31 to 41% SiOz, 18 to 33% PbO, 12 to 25% BaO, the total PbO and Ba being 40 to 50%, to ZrOz,

tion selected from the group consisting of up to 8% K20, up to 9% NazO and up to 2.5 LizO, the selected alkali metal oxide including an oxide selected from the group consisting of NazO and Liz0,"the total of the alkali metal oxides being 6 to 9%, 3 to 6% E203, the

i t proportion selected from the group consisting of up to r I r Zr02, at least one alkali metal oxide in the indicated 4 8% K20, up to 9% NazO and up to 2.5% LizO, the selected alkali metal oxide including an oxide selected from the group consisting of NazO and Li20, the total of the alkali metal oxides being 6 to 9%, 3 to 6% B203,

and up to 2% TiO 4. A glass having approximately the following composition by weight:

Percent Silica (SiOz) 37 Lead oxide (PbO) 22.7 Barium oxide (BaO) 22. Zirconium oxide (Zr02) 7 Sodium oxide (Na) 6.8 Boric oxide (B203) 3.5

Titanium oxide (TiOz) 1 at least one alkali metal oxide in the indicated propor- I total of the essential constituents being at least 98%,

5. A glass having approximately the following composition by weight:

. .Percent Silica (SiOz) 38.5 Lead oxide (PbO) 30.2 Barium oxide (BaO) 15 Zirconium oxide (ZrOz) 5.2 Sodium oxide (NazO) 7.1 Boric oxide .(BzOs) 4 Rx Lenses, American Optical Company Catalog, page 64, published in 1950. v 

1. A CALCIUM-FREE GLASS COMPRISING ESSENTIALLY BY WEIGHT 31 TO 41% SIO2, 18 TO 33% PBO, 12 TO 25% BAO, THE TOTAL PBO AND BAO BEING 40 TO 50%, 5 TO 10% ZRO2, AT LEAST ONE ALKALI METAL OXIDE IN THE INDICATED PROPORTION SELECTED FROM THE GROUP CONSISTING OF UP TO 8% K2O, UP TO 9% NA2O AND UP TO 2.5% LI2O, THE SELECTED ALKALI METAL OXIDE INCLUDING AN OXIDE SELECTED FROM THE GROUP CONSISTING OF NA2O AND LI2O, THE TOTAL OF THE ALKALI METAL OXIDES BEING 6 TO 9%, AND 3 TO 6% B2O3, THE TOTAL OF THE ESSENTIAL CONSTITUENTS BEING AT LEAST 98%. 